Method and system for forming high accuracy patterns using charged particle beam lithography

ABSTRACT

A method and system for fracturing or mask data preparation for charged particle beam lithography are disclosed in which accuracy and/or edge slope of a pattern formed on a surface by a set of charged particle beam shots is enhanced by use of partially-overlapping shots. In some embodiments, dosages of the shots may vary with respect to each other before proximity effect correction. Particle beam simulation may be used to calculate the pattern and the edge slope. Enhanced edge slope can improve critical dimension (CD) variation and line-edge roughness (LER) of the pattern produced on the surface.

RELATED APPLICATIONS

This application: 1) claims priority to U.S. Provisional PatentApplication Ser. No. 61/392,477 filed on Oct. 13, 2010 and entitled“Method for Integrated Circuit Manufacturing and Mask Data PreparationUsing Curvilinear Patterns”; and 2) is related to U.S. patentapplication Ser. No. 13/168,953 filed on Jun. 25, 2011 entitled “Methodand System for Forming Patterns with Charged Particle Beam Lithography”;both of which are hereby incorporated by reference for all purposes.

BACKGROUND OF THE DISCLOSURE

The present disclosure is related to lithography, and more particularlyto the design and manufacture of a surface which may be a reticle, awafer, or any other surface, using charged particle beam lithography.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, optical lithography may be used to fabricate thesemiconductor devices. Optical lithography is a printing process inwhich a lithographic mask or photomask manufactured from a reticle isused to transfer patterns to a substrate such as a semiconductor orsilicon wafer to create the integrated circuit (I.C.). Other substratescould include flat panel displays, holographic masks, or even otherreticles. While conventional optical lithography uses a light sourcehaving a wavelength of 193 nm, extreme ultraviolet (EUV) or X-raylithography are also considered types of optical lithography. Thereticle or multiple reticles may contain a circuit pattern correspondingto an individual layer of the integrated circuit, and this pattern canbe imaged onto a certain area on the substrate that has been coated witha layer of radiation-sensitive material known as photoresist or resist.Once the patterned layer is transferred the layer may undergo variousother processes such as etching, ion-implantation (doping),metallization, oxidation, and polishing. These processes are employed tofinish an individual layer in the substrate. If several layers arerequired, then the whole process or variations thereof will be repeatedfor each new layer. Eventually, a combination of multiples of devices orintegrated circuits will be present on the substrate. These integratedcircuits may then be separated from one another by dicing or sawing andthen may be mounted into individual packages. In the more general case,the patterns on the substrate may be used to define artifacts such asdisplay pixels, holograms, or magnetic recording heads.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, non-optical methods may be used to transfer apattern on a lithographic mask to a substrate such as a silicon wafer.Nanoimprint lithography (NIL) is an example of a non-optical lithographyprocess. In nanoimprint lithography, a lithographic mask pattern istransferred to a surface through contact of the lithography mask withthe surface.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, maskless direct write may also be used to fabricatethe semiconductor devices. Maskless direct write is a printing processin which charged particle beam lithography is used to transfer patternsto a substrate such as a semiconductor or silicon wafer to create theintegrated circuit. Other substrates could include flat panel displays,imprint masks for nano-imprinting, or even reticles. Desired patterns ofa layer are written directly on the surface, which in this case is alsothe substrate. Once the patterned layer is transferred the layer mayundergo various other processes such as etching, ion-implantation(doping), metallization, oxidation, and polishing. These processes areemployed to finish an individual layer in the substrate. If severallayers are required, then the whole process or variations thereof willbe repeated for each new layer. Some of the layers may be written usingoptical lithography while others may be written using maskless directwrite to fabricate the same substrate. Also, some patterns of a givenlayer may be written using optical lithography, and other patternswritten using maskless direct write. Eventually, a combination ofmultiples of devices or integrated circuits will be present on thesubstrate. These integrated circuits are then separated from one anotherby dicing or sawing and then mounted into individual packages. In themore general case, the patterns on the surface may be used to defineartifacts such as display pixels, holograms, or magnetic recordingheads.

Two common types of charged particle beam lithography are variableshaped beam (VSB) and character projection (CP). These are bothsub-categories of shaped beam charged particle beam lithography, inwhich a precise electron beam is shaped and steered so as to expose aresist-coated surface, such as the surface of a wafer or the surface ofa reticle. In VSB, these shapes are simple shapes, usually limited torectangles of certain minimum and maximum sizes and with sides which areparallel to the axes of a Cartesian coordinate plane (i.e. of“manhattan” orientation), and 45 degree right triangles (i.e. triangleswith their three internal angles being 45 degrees, 45 degrees, and 90degrees) of certain minimum and maximum sizes. At predeterminedlocations, doses of electrons are shot into the resist with these simpleshapes. The total writing time for this type of system increases withthe number of shots. In character projection (CP), there is a stencil inthe system that has in it a variety of apertures or characters which maybe complex shapes such as rectilinear, arbitrary-angled linear,circular, nearly circular, annular, nearly annular, oval, nearly oval,partially circular, partially nearly circular, partially annular,partially nearly annular, partially nearly oval, or arbitrarycurvilinear shapes, and which may be a connected set of complex shapesor a group of disjointed sets of a connected set of complex shapes. Anelectron beam can be shot through a character on the stencil toefficiently produce more complex patterns on the reticle. In theory,such a system can be faster than a VSB system because it can shoot morecomplex shapes with each time-consuming shot. Thus, an E-shaped patternshot with a VSB system takes four shots, but the same E-shaped patterncan be shot with one shot with a character projection system. Note thatVSB systems can be thought of as a special (simple) case of characterprojection, where the characters are just simple characters, usuallyrectangles or 45-45-90 degree triangles. It is also possible topartially expose a character. This can be done by, for instance,blocking part of the particle beam. For example, the E-shaped patterndescribed above can be partially exposed as an F-shaped pattern or anI-shaped pattern, where different parts of the beam are cut off by anaperture. This is the same mechanism as how various sized rectangles canbe shot using VSB. In this disclosure, partial projection is used tomean both character projection and VSB projection.

As indicated, in optical lithography the lithographic mask or reticlecomprises geometric patterns corresponding to the circuit components tobe integrated onto a substrate. The patterns used to manufacture thereticle may be generated utilizing computer-aided design (CAD) softwareor programs. In designing the patterns the CAD program may follow a setof pre-determined design rules in order to create the reticle. Theserules are set by processing, design, and end-use limitations. An exampleof an end-use limitation is defining the geometry of a transistor in away in which it cannot sufficiently operate at the required supplyvoltage. In particular, design rules can define the space tolerancebetween circuit devices or interconnect lines. The design rules are, forexample, used to ensure that the circuit devices or lines do notinteract with one another in an undesirable manner. For example, thedesign rules are used so that lines do not get too close to each otherin a way that may cause a short circuit. The design rule limitationsreflect, among other things, the smallest dimensions that can bereliably fabricated. When referring to these small dimensions, oneusually introduces the concept of a critical dimension. These are, forinstance, defined as the smallest width of a line or the smallest spacebetween two lines, those dimensions requiring exquisite control.

One goal in integrated circuit fabrication by optical lithography is toreproduce the original circuit design on the substrate by use of thereticle. Integrated circuit fabricators are always attempting to use thesemiconductor wafer real estate as efficiently as possible. Engineerskeep shrinking the size of the circuits to allow the integrated circuitsto contain more circuit elements and to use less power. As the size ofan integrated circuit critical dimension is reduced and its circuitdensity increases, the critical dimension of the circuit pattern orphysical design approaches the resolution limit of the optical exposuretool used in conventional optical lithography. As the criticaldimensions of the circuit pattern become smaller and approach theresolution value of the exposure tool, the accurate transcription of thephysical design to the actual circuit pattern developed on the resistlayer becomes difficult. To further the use of optical lithography totransfer patterns having features that are smaller than the lightwavelength used in the optical lithography process, a process known asoptical proximity correction (OPC) has been developed. OPC alters thephysical design to compensate for distortions caused by effects such asoptical diffraction and the optical interaction of features withproximate features. OPC includes all resolution enhancement technologiesperformed with a reticle.

OPC may add sub-resolution lithographic features to mask patterns toreduce differences between the original physical design pattern, thatis, the design, and the final transferred circuit pattern on thesubstrate. The sub-resolution lithographic features interact with theoriginal patterns in the physical design and with each other andcompensate for proximity effects to improve the final transferredcircuit pattern. One feature that is used to improve the transfer of thepattern is a sub-resolution assist feature (SRAF). Another feature thatis added to improve pattern transference is referred to as “serifs”.Serifs are small features that can be positioned on a corner of apattern to sharpen the corner in the final transferred image. It isoften the case that the precision demanded of the surface manufacturingprocess for SRAFs are less than those for patterns that are intended toprint on the substrate, often referred to as main features. Serifs are apart of a main feature. As the limits of optical lithography are beingextended far into the sub-wavelength regime, the OPC features must bemade more and more complex in order to compensate for even more subtleinteractions and effects. As imaging systems are pushed closer to theirlimits, the ability to produce reticles with sufficiently fine OPCfeatures becomes critical. Although adding serifs or other OPC featuresto a mask pattern is advantageous, it also substantially increases thetotal feature count in the mask pattern. For example, adding a serif toeach of the corners of a square using conventional techniques adds eightmore rectangles to a mask or reticle pattern. Adding OPC features is avery laborious task, requires costly computation time, and results inmore expensive reticles. Not only are OPC patterns complex, but sinceoptical proximity effects are long range compared to minimum line andspace dimensions, the correct OPC patterns in a given location dependsignificantly on what other geometry is in the neighborhood. Thus, forinstance, a line end will have different size serifs depending on whatis near it on the reticle. This is even though the objective might be toproduce exactly the same shape on the wafer. These slight but criticalvariations are important and have prevented others from being able toform reticle patterns. It is conventional to discuss the OPC-decoratedpatterns to be written on a reticle in terms of main features, that isfeatures that reflect the design before OPC decoration, and OPCfeatures, where OPC features might include serifs, jogs, and SRAF. Toquantify what is meant by slight variations, a typical slight variationin OPC decoration from neighborhood to neighborhood might be 5% to 80%of a main feature size. Note that for clarity, variations in the designof the OPC are what is being referenced. Manufacturing variations, suchas line-edge roughness and corner rounding, will also be present in theactual surface patterns. When these OPC variations produce substantiallythe same patterns on the wafer, what is meant is that the geometry onthe wafer is targeted to be the same within a specified error, whichdepends on the details of the function that that geometry is designed toperform, e.g., a transistor or a wire. Nevertheless, typicalspecifications are in the 2%-50% of a main feature range. There arenumerous manufacturing factors that also cause variations, but the OPCcomponent of that overall error is often in the range listed. OPC shapessuch as sub-resolution assist features are subject to various designrules, such as a rule based on the size of the smallest feature that canbe transferred to the wafer using optical lithography. Other designrules may come from the mask manufacturing process or, if a characterprojection charged particle beam writing system is used to form thepattern on a reticle, from the stencil manufacturing process. It shouldalso be noted that the accuracy requirement of the SRAF features on themask may be lower than the accuracy requirements for the main featureson the mask. As process nodes continue to shrink, the size of thesmallest SRAFs on a photomask also shrinks For example, at the 20 nmlogic process node, 40 nm to 60 nm SRAFs are needed on the mask for thehighest precision layers.

Inverse lithography technology (ILT) is one type of OPC technique. ILTis a process in which a pattern to be formed on a reticle is directlycomputed from a pattern which is desired to be formed on a substratesuch as a silicon wafer. This may include simulating the opticallithography process in the reverse direction, using the desired patternon the substrate as input. ILT-computed reticle patterns may be purelycurvilinear—i.e. completely non-rectilinear—and may include circular,nearly circular, annular, nearly annular, oval and/or nearly ovalpatterns. Since curvilinear patterns are difficult and expensive to formon a reticle using conventional techniques, rectilinear approximationsof the curvilinear patterns may be used. In this disclosure ILT, OPC,source mask optimization (SMO), and computational lithography are termsthat are used interchangeably.

EUV optical lithography has a much higher resolution than conventionaloptical lithography. The very high resolution of EUV significantlyreduces the need for OPC processing, resulting in lower mask complexityfor EUV than for 193 nm optical lithography. However, because of thevery high resolution of EUV, imperfections in a photomask, such asexcessive line edge roughness (LER), will be transferred to the wafer.Therefore, the accuracy requirements for EUV masks are higher than thosefor conventional optical lithography. Additionally, even though EUV maskshapes are not complicated by the addition of complex SRAFs or serifsrequired for conventional 193 nm lithography, EUV mask shapes arecomplicated by an addition of some complexities unique to EUVmanufacturing. Of particular relevance in writing patterns on masks forEUV lithography is mid-range scattering of charged particles such aselectrons, which may affect a radius of about 2 um. This midrangescattering introduces a new consideration for mask data preparation,because for the first time the influence from neighboring patterns hassignificant impact on the shape that a particular pattern would castonto the mask surface. Previously, when exposing masks for use withconventional 193 nm lithography, the short-range scattering affectedonly the pattern being written, and the long-range scattering had alarge enough effective range that only the size of a pattern, and notits detailed shape, was affected, making it possible to make correctionsby only using dose modulation. In addition, since EUV processing ofwafers is more expensive, it is desirable to reduce or eliminatemultiple patterning. Multiple patterning is used in conventional opticallithography to allow exposure of small features by exposing patterns forone layer of wafer processing using multiple masks, each of whichcontains a portion of the layer pattern. Reducing or eliminatingmultiple exposures requires the single mask to contain more finepatterns. For example, a series of collinear line segments may bedouble-patterned by first drawing a long line, then cutting the lineinto line segments by a second mask in conventional lithography. Thesame layer written with a single mask, such as for EUV lithography,would require a mask containing many smaller line segments. The need towrite larger numbers of finer patterns on a single mask, each patternneeding to be more accurate, increases the need for precision on EUVmasks.

There are a number of technologies used for forming patterns on areticle, including using optical lithography or charged particle beamlithography. The most commonly used system is the variable shaped beam(VSB), where, as described above, doses of electrons with simple shapessuch as manhattan rectangles and 45-degree right triangles expose aresist-coated reticle surface. In conventional mask writing, the dosesor shots of electrons are conventionally designed to avoid overlapwherever possible, so as to greatly simplify calculation of how theresist on the reticle will register the pattern. Similarly, the set ofshots is designed so as to completely cover the pattern area that is tobe formed on the reticle.

Reticle writing for the most advanced technology nodes typicallyinvolves multiple passes of charged particle beam writing, a processcalled multi-pass exposure, whereby the given shape on the reticle iswritten and overwritten. Typically, two to four passes are used to writea reticle to average out precision errors in the charged particle beamwriter, allowing the creation of more accurate photomasks. Alsotypically, the list of shots, including the dosages, is the same forevery pass. In one variation of multi-pass exposure, the lists of shotsmay vary among exposure passes, but the union of the shots in anyexposure pass covers the same area. Multi-pass writing can reduceover-heating of the resist coating the surface. Multi-pass writing alsoaverages out random errors of the charged particle beam writer.Multi-pass writing using different shot lists for different exposurepasses can also reduce the effects of certain systemic errors in thewriting process.

Current optical lithography writing machines typically reduce thephotomask pattern by a factor of four during the optical lithographicprocess. Therefore, patterns formed on a reticle or mask must be fourtimes larger than the size of the desired pattern on the substrate orwafer.

Current-technology charged particle beam writers, using conventionaltechniques, can resolve features as small as 100 nm. For featuressmaller than 100 nm, however, conventional writing techniques may failto accurately resolve features. Additionally, manufacturing variationmay produce unacceptable LER and critical dimension (CD) variation. Thiscan be a problem for both conventional optical lithography, where OPCmay produce SRAF's having mask dimensions smaller than 100 nm, and forEUV lithography, where the main mask patterns may be smaller than 100 nmand where mask specifications may be tighter than for masks used forconventional optical lithography.

SUMMARY OF THE DISCLOSURE

A method and system for fracturing or mask data preparation for chargedparticle beam lithography are disclosed in which accuracy and/or dosemargin of a pattern formed on a surface by a set of charged particlebeam shots is enhanced by use of partially-overlapping shots. In someembodiments, dosages of the shots may vary with respect to each otherbefore proximity effect correction. Particle beam simulation may be usedto calculate the pattern and the dose margin. Enhanced dose margin canimprove critical dimension (CD) variation and line-edge roughness (LER)of the pattern produced on the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a character projection charged particlebeam system;

FIG. 2A illustrates an example of a cross-sectional dosage graph,showing the registered pattern widths for each of two resist thresholds;

FIG. 2B illustrates an example of a cross-sectional dosage graph similarto FIG. 2A, but with a higher dosage edge slope than in FIG. 2A;

FIG. 3A illustrates an example of a desired 100 nm line-end pattern tobe formed on a reticle;

FIG. 3B illustrates an example of a simulated pattern formed using shotsgenerated by fracturing the pattern of FIG. 3A using conventionaltechniques;

FIG. 4A illustrates an example of a desired 80 nm line-end pattern to beformed on a reticle;

FIG. 4B illustrates an example of a simulated pattern formed using shotsgenerated by fracturing the pattern of FIG. 4A using conventionaltechniques;

FIG. 5A illustrates an example of a desired 60 nm line-end pattern to beformed on a reticle;

FIG. 5B illustrates an example of a simulated pattern formed using shotsgenerated by fracturing the pattern of FIG. 5A using conventionaltechniques;

FIG. 6 illustrates various examples of groups of shots that may be usedto form a 80 nm line-end pattern;

FIG. 7 illustrates simulated patterns formed by the various shot groupsof FIG. 6;

FIG. 8A illustrates an example of a group of rectangular patterns to beformed on a surface;

FIG. 8B illustrates an example of how the patterns of FIG. 8A may beformed on a surface using conventional non-overlapping VSB shots, in thepresence of mid-range scattering;

FIG. 9A illustrates an example of a set of overlapping VSB shots thatmay be used to form the patterns of FIG. 8A on a surface;

FIG. 9B illustrates an example of a pattern that may be formed on asurface from the shots of FIG. 9A;

FIG. 10 illustrates a conceptual flow diagram of how to prepare asurface, such as a reticle, for use in fabricating a substrate such asan integrated circuit on a silicon wafer using optical lithography;

FIG. 11A illustrates a conceptual flow diagram of one method ofcombining model-based and conventional fracturing in the same design;and

FIG. 11B illustrates a conceptual flow diagram of another method ofcombining model-based and conventional fracturing in the same design.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure describes a method for enhancing the accuracy ofcharged particle beam exposure by use of overlapping shots. The presentinvention enhances the ability of charged particle beam systems toaccurately produce patterns smaller than 100 nm on a reticle, withacceptable CD variation and LER in light of manufacturing variation.Additionally, the present invention expands the process window ofmanufacturing variations under which these accurate patterns may beproduced.

Referring now to the drawings, wherein like numbers refer to like items,FIG. 1 illustrates an embodiment of a conventional lithography system100, such as a charged particle beam writer system, in this case anelectron beam writer system, that employs character projection tomanufacture a surface 130. The electron beam writer system 100 has anelectron beam source 112 that projects an electron beam 114 toward anaperture plate 116. The plate 116 has an aperture 118 formed thereinwhich allows the electron beam 114 to pass. Once the electron beam 114passes through the aperture 118 it is directed or deflected by a systemof lenses (not shown) as electron beam 120 toward another rectangularaperture plate or stencil mask 122. The stencil 122 has formed therein anumber of openings or apertures 124 that define various types ofcharacters 126, which may be complex characters. Each character 126formed in the stencil 122 may be used to form a pattern 148 on a surface130 of a substrate 132, such as a silicon wafer, a reticle or othersubstrate. In partial exposure, partial projection, partial characterprojection, or variable character projection, electron beam 120 may bepositioned so as to strike or illuminate only a portion of one of thecharacters 126, thereby forming a pattern 148 that is a subset ofcharacter 126. For each character 126 that is smaller than the size ofthe electron beam 120 defined by aperture 118, a blanking area 136,containing no aperture, is designed to be adjacent to the character 126,so as to prevent the electron beam 120 from illuminating an unwantedcharacter on stencil 122. An electron beam 134 emerges from one of thecharacters 126 and passes through an electromagnetic or electrostaticreduction lens 138 which reduces the size of the pattern from thecharacter 126. In commonly available charged particle beam writersystems, the reduction factor is between 10 and 60. The reduced electronbeam 140 emerges from the reduction lens 138, and is directed by aseries of deflectors 142 onto the surface 130 as the pattern 148, whichis depicted as being in the shape of the letter “H” corresponding tocharacter 126A. The pattern 148 is reduced in size compared to thecharacter 126A because of the reduction lens 138. The pattern 148 isdrawn by using one shot of the electron beam system 100. This reducesthe overall writing time to complete the pattern 148 as compared tousing a variable shape beam (VSB) projection system or method. Althoughone aperture 118 is shown being formed in the plate 116, it is possiblethat there may be more than one aperture in the plate 116. Although twoplates 116 and 122 are shown in this example, there may be only oneplate or more than two plates, each plate comprising one or moreapertures.

In conventional charged particle beam writer systems the reduction lens138 is calibrated to provide a fixed reduction factor. The reductionlens 138 and/or the deflectors 142 also focus the beam on the plane ofthe surface 130. The size of the surface 130 may be significantly largerthan the maximum beam deflection capability of the deflection plates142. Because of this, patterns are normally written on the surface in aseries of stripes. Each stripe contains a plurality of sub-fields, wherea sub-field is within the beam deflection capability of the deflectionplates 142. The electron beam writer system 100 contains a positioningmechanism 150 to allow positioning the substrate 132 for each of thestripes and sub-fields. In one variation of the conventional chargedparticle beam writer system, the substrate 132 is held stationary whilea sub-field is exposed, after which the positioning mechanism 150 movesthe substrate 132 to the next sub-field position. In another variationof the conventional charged particle beam writer system, the substrate132 moves continuously during the writing process. In this variationinvolving continuous movement, in addition to deflection plates 142,there may be another set of deflection plates (not shown) to move thebeam at the same speed and direction as the substrate 132 is moved.

The minimum size pattern that can be projected with reasonable accuracyonto a surface 130 is limited by a variety of short-range physicaleffects associated with the electron beam writer system 100 and with thesurface 130, which normally comprises a resist coating on the substrate132. These effects include forward scattering, Coulomb effect, andresist diffusion. Beam blur is a term used to include all of theseshort-range effects. The most modern electron beam writer systems canachieve an effective beam blur radius or β_(f) in the range of 20 nm to30 nm. Forward scattering may constitute one quarter to one half of thetotal beam blur. Modern electron beam writer systems contain numerousmechanisms to reduce each of the constituent pieces of beam blur to aminimum. Some electron beam writer systems may allow the beam blur to bevaried during the writing process, from the minimum value available onan electron beam writing system to one or more larger values.

The shot dosage of a charged particle beam writer such as an electronbeam writer system is a function of the intensity of the beam source 112and the exposure time for each shot. Typically the beam intensityremains fixed, and the exposure time is varied to obtain variable shotdosages. The exposure time may be varied to compensate for variouslong-range effects such as back scatter and fogging in a process calledproximity effect correction (PEC). Electron beam writer systems usuallyallow setting an overall dosage, called a base dosage, that affects allshots in an exposure pass. Some electron beam writer systems performdosage compensation calculations within the electron beam writer systemitself, and do not allow the dosage of each shot to be assignedindividually as part of the input shot list, the input shots thereforehaving unassigned shot dosages. In such electron beam writer systems allshots have the base dosage, before proximity effects correction. Otherelectron beam writer systems do allow dosage assignment on ashot-by-shot basis. In electron beam writer systems that allowshot-by-shot dosage assignment, the number of available dosage levelsmay be 64 to 4096 or more, or there may be a relatively few availabledosage levels, such as 3 to 8 levels. Some embodiments of the currentinvention are targeted for use with charged particle beam writingsystems which allow assignment of dosage levels.

The mechanisms within electron beam writers have a relatively coarseresolution for calculations. As such, mid-range corrections such as maybe required for EUV masks in the range of 2 um cannot be computedaccurately by current electron beam writers.

Conventionally, shots are designed so as to completely cover an inputpattern with rectangular shots, while avoiding shot overlap whereverpossible. Also, all shots are designed to have a normal dosage, which isa dosage at which a relatively large rectangular shot, in the absence oflong-range effects, will produce a pattern on the surface which is thesame size as is the shot size.

In exposing, for example, a repeated pattern on a surface using chargedparticle beam lithography, the size of each pattern instance, asmeasured on the final manufactured surface, will be slightly different,due to manufacturing variations. The amount of the size variation is anessential manufacturing optimization criterion. In current mask masking,a root mean square (RMS) variation of no more than 1 nm (1 sigma) inpattern size may be desired. More size variation translates to morevariation in circuit performance, leading to higher design margins beingrequired, making it increasingly difficult to design faster, lower-powerintegrated circuits. This variation is referred to as critical dimension(CD) variation. A low CD variation is desirable, and indicates thatmanufacturing variations will produce relatively small size variationson the final manufactured surface. In the smaller scale, the effects ofa high CD variation may be observed as line edge roughness (LER). LER iscaused by each part of a line edge being slightly differentlymanufactured, leading to some waviness in a line that is intended tohave a straight edge. CD variation is, among other things, inverselyrelated to the slope of the dosage curve at the resist threshold, whichis called edge slope. Therefore, edge slope, or dose margin, is acritical optimization factor for particle beam writing of surfaces. Inthis disclosure, edge slope and dose margin are terms that are usedinterchangeably.

With conventional fracturing, without shot overlap, gaps or dosemodulation, the dose margin of the written shapes is consideredimmutable: that is, there is no opportunity to improve dose margin by achoice of fracturing options. In modern practice, the avoidance of verynarrow shots called slivers is an example of a practical rule-basedmethod that helps to optimize the shot list for dose margin.

In a fracturing environment where overlapping shots and dose-modulatedshots can be generated, there is both a need and an opportunity tooptimize for dose margin. The additional flexibility in shotcombinations allowed by use of shot overlap and dose modulation allowsgeneration of many fracturing solutions that appear to generate thetarget mask shapes on the surface, but do so only under perfectmanufacturing conditions. The use of overlapping shots anddose-modulated shots therefore creates incentive to address the issue ofdose margin and its improvement.

FIG. 2A-B illustrates how critical dimension variation can be reduced byexposing the pattern on the resist so as to produce a relatively highedge slope in the exposure or dosage curve. FIG. 2A illustrates across-sectional dosage curve 202, where the x-axis shows thecross-sectional distance through an exposed pattern—such as the distanceperpendicular to two of the pattern's edges—and the y-axis shows thedosage received by the resist. A pattern is registered by the resistwhere the received dosage is higher than a threshold. Two thresholds areillustrated in FIG. 2A, illustrating the effect of a variation in resistsensitivity. The higher threshold 204 causes a pattern of width 214 tobe registered by the resist. The lower threshold 206 causes a pattern ofwidth 216 to be registered by the resist, where width 216 is greaterthan width 214. FIG. 2B illustrates another cross-sectional dosage curve222. Two thresholds are illustrated, where threshold 224 is the same asthreshold 204 of FIG. 2A, and threshold 226 is the same as threshold 206of FIG. 2A. The slope of dosage curve 222 is higher in the vicinity ofthe two thresholds than is the slope of dosage curve 202. For dosagecurve 222, the higher threshold 224 causes a pattern of width 234 to beregistered by the resist. The lower threshold 226 causes a pattern ofwidth 236 to be registered by the resist. As can be seen, the differencebetween width 236 and width 234 is less than the difference betweenwidth 216 and width 214, due to the higher edge slope of dosage curve222 compared to dosage curve 202. If the resist-coated surface is areticle, then the lower sensitivity of curve 222 to variation in resistthreshold can cause the pattern width on a photomask manufactured fromthe reticle to be closer to the target pattern width, thereby increasingthe yield of usable integrated circuits when the photomask is used totransfer a pattern to a substrate such as a silicon wafer. Similarimprovement in tolerance to variation in dose for each shot is observedfor dose curves with higher edge slopes. Achieving a relatively higheredge slope such as in dosage curve 222 is therefore desirable.

FIG. 3A illustrates an example of a designed pattern 302. Pattern 302 isdesigned to have a constant width 306, the width being 100 nm. Pattern302 comprises a line-end 304. FIG. 3B illustrates an example of asimulated pattern 312 that may be formed on a surface using aconventional VSB shot, where the VSB shot is a 100 nm wide rectangle,and of a normal dosage. As can be seen in FIG. 3B, the line-end portion314 of pattern 312 has rounded corners, due to beam blur caused by thephysical limitations of the charged particle beam writer. Additionally,the exposed pattern has a poor edge slope in sections 316 and 318 of thepattern perimeter. This edge slope may be determined, for example, usingparticle beam simulation. The portions 316 and 318 of the pattern 312may cause an undesirably-large variation in size due to manufacturingvariation. The line-end 314, in its center section, however, is thedesired length—i.e. having the same y-coordinate as the designedline-end 304.

FIG. 4A illustrates an example of a designed pattern 402. Pattern 402 isdesigned to have a constant width 406 of 80 nm. Pattern 402 comprises aline-end 404, where the y-coordinate of the line-end 404 is shown byreference line 408. FIG. 4B illustrates an example of a simulatedpattern 412 that may be formed on a surface using a conventional VSBshot, where the VSB shot is 80 nm wide, and of a normal dosage. As withpattern 312, the line-end portion 414 of pattern 412 has rounded cornersdue to beam blur. Also, the portions 416 and 418 of the perimeter ofpattern 412 have poor edge slope. As can be seen, the portions 416 and418 of the perimeter of pattern 412 having poor edge slope are largerthan the portions 316 and 318 of pattern 312 which have poor edge slope.This is due to the narrower 80 nm width of pattern 402 compared to the100 nm width of pattern 302. Additionally, the y-coordinate of formedline-end 414 is larger than the y-coordinate of the reference line 408,meaning that pattern 412 has line-end shortening, which can affect theperformance and/or functionality of an integrated circuit fabricatedusing a mask containing pattern 412.

FIG. 5A illustrates an example of a designed pattern 502. Pattern 502 isdesigned to have a constant width 508 of 60 nm. Pattern 502 comprises aline-end 504, where the y-coordinate of line-end 504 is shown byreference line 506. FIG. 5B illustrates an example of a pattern 512 thatmay be formed on a surface using a conventional VSB shot, where the VSBshot is 60 nm wide, and of a normal dosage. As can be seen, the line-endportion 514 of pattern 512 is very rounded. There is also line-endshortening—the minimum y-coordinate of pattern 512 is greater than they-coordinate of reference line 506. Additionally, the perimeter region518 of pattern 512 has a poor edge slope, affecting the entire line-end514.

The patterns of FIGS. 3B, 4B and 5B illustrate how formation of patternsof 80 nm width and below may have line-end shortening, and may also haverounded corners with poor edge slope, when formed with conventional VSBshots.

FIG. 6 illustrates various methods of fracturing a pattern to enhancethe quality of the pattern formed on a surface such as a reticle. Shape602 illustrates a designed line-end pattern, the pattern 602 having awidth 604 of 80 nm. The pattern 602 comprises a line-end 606. Dashedline 608 denotes the y-coordinate of line-end 606. FIG. 6 pattern 612illustrates one prior art method of fracturing pattern 602 to improvethe quality of the formed pattern on a surface, compared with FIG. 4Bpattern 412. Pattern 612 illustrates a single VSB shot, where the shotsize has been expanded in the negative y-dimension, so that the minimumy-coordinate of the shot is 7 nm less than reference y-coordinate 608.The dose of shot 612 is a normal dose. FIG. 7 pattern 712 illustrates asimulated shape of the shot 612. The line end of pattern 712 has roundedcorners, and also has perimeter regions 714 and 716 in which the edgeslope of the pattern is too low.

FIG. 6 also illustrates three groups of VSB shots, group 622, group 632and group 642, which can form the pattern 602. Shot group 632 and shotgroup 642 exemplify one embodiment of the current invention while shotgroup 622 represents a prior art method. Shot group 622 consists of shot624 and shot 626, which do not overlap each other. Shot 624 is shot at1.2 times a normal dose, before long-range PEC, and shot 626 is shot ata normal dose. The width 628 of shot 624 is less than 604, and iscalculated so as to produce a pattern of width 604 on the surface withthe larger-than-normal dosage. Shot 626, as can be seen, is extended inthe negative and positive x-directions beyond the dimensions of shot 624and also beyond the dimensions of pattern 602. FIG. 7 pattern 722illustrates the simulated pattern produced by shot group 622. Theline-end corners 724 of pattern 722 have a higher edge slope thanpattern 712, with no part of the corner having a too-small edge slope.Additionally, though not illustrated, the higher-than-normal dose ofshot 624 improves the edge slope on the left and right sides of pattern722 compared to pattern 712. One method of determining the shots of shotgroup 622 is through model-based fracturing, which is the use ofsimulation, such as charged particle beam simulation, to determine a setof shots which can form a desired pattern on a resist-coated surface, bydetermining through simulation the pattern which will be produced on thesurface from a given set of one or more shots, where some or all of theshots may have non-normal dosages. Alternatively, the shots of shotgroup 622 may be determined through rule-based methods. Model-basedfracturing, although relatively more compute-intensive than rule-basedfracturing, may determine a shot list that will produce a more accuratepattern on the surface, compared to a shot list determined usingrule-based methods.

FIG. 6 shot group 632 illustrates an exemplary method of fracturingpattern 602 according to one embodiment of the current invention. Shotgroup 632 consists of shot 634, shot 636 and shot 638. Shots 636 and 638are illustrated with shading for improved clarity. Shot 634 is shot at ahigher-than-normal dose, for example 1.2 times normal dose, and thewidth of shot 634 is calculated so as to produce a pattern of width 604on a surface. Shot 636 and shot 638 both overlap shot 634, and bothextend below reference y-coordinate 608. The overlap between, forexample, shot 634 and 636 is a partial overlap, meaning that the area ofintersection between shot 634 and shot 636 is different than eithershot. Shot 636 and shot 638 are shot at a normal dose in this example.FIG. 7 pattern 732 illustrates a simulated pattern from shot group 632.Compared to pattern 722, pattern 732 exhibits less corner rounding, butalso has worse edge slope on the corners, with the edge slope being lessthan the minimum acceptable value in perimeter regions 734 and 736. Shotgroup 632 illustrates how use of overlapping shots and other-than-normaldosages may allow patterns to be formed with higher-fidelity than usingconventional non-overlapping shots with normal dosages.

FIG. 6 shot group 642 illustrates another example for fracturing pattern602 according to the current invention, using partially overlappingshots. Shot group 642 consists of shots 644, 646, 648 and 650. Shots646, 648 and 650 are illustrated with shading for improved clarity. Likeshots 624 and 634, shot 644 uses a higher-than-normal dose such as of1.2× normal. Shots 646, 648 and 650 use a normal dose in this example.Shot 650 overlaps shot 644. Shots 646 and 648 extend beyond referencey-coordinate 608. FIG. 7 pattern 742 illustrates a simulated patternfrom shot group 642. The corners 744 of the pattern 742 line-end areless rounded than, for example, the corers of pattern 722. Additionally,the edge slope in the corner region is higher-than-minimum at alllocations. Like shot group 632, shot group 642 illustrates how use ofoverlapping shots combined with other-than-normal dosages may allowpatterns to be formed with higher-fidelity than with conventionalmethods or prior art methods such as illustrated with the method of shot612.

The solution described above and illustrated in FIG. 6 shot groups 632and 642 may be implemented even using a charged particle beam systemthat does not allow dosage assignment for individual shots. In oneembodiment of the present invention, a small number of dosages may beselected, for example two dosages such as 1.0× normal and 1.2× normal,and shots for each of these two dosages may be separated and exposed intwo separate exposure passes, where the base dosage for one exposurepass is 1.0× normal and the base dosage for the other exposure pass is1.2× normal. For example, in FIG. 6 shot group 632, shot 636 and shot638 may be assigned to a first exposure pass using a base dosage of 1.0×normal dosage, and shot 634 may be assigned to a second exposure passusing a base dosages of 1.2× normal dosage. In this embodiment, theunion of shots for any exposure pass will be different than the union ofshots for all of the exposure passes combined.

In other embodiments of the current invention, sensitivity to types ofmanufacturing variation other than dosage variation may be reduced byusing overlapping shots. Beam blur variation is an example of anothertype of manufacturing variation. Additionally, the methods of thecurrent invention may also be practiced using complex characterprojection (CP) shots, or with a combination of complex CP and VSBshots.

FIG. 8A illustrates an example of a group of rectangular patterns 800 tobe formed on a surface. The group of patterns 800 comprises six completerectangles, including rectangle 802, rectangle 804, rectangle 806,rectangle 808, rectangle 810 and rectangle 812. Additionally, portionsof four additional rectangles are illustrated: rectangle 814, rectangle816, rectangle 818 and rectangle 820. As can be seen, the rectangles arearranged in a regular pattern with columns, where adjacent columns areseparated by a space 830, and where adjacent rectangles within a columnare separated by a space 832.

Pattern group 800 can be written to a surface using conventionalnon-overlapping VSB shots, using one VSB shot for each pattern inpattern group 800. FIG. 8A can therefore also be viewed as a group ofshots 800, comprising shots 802, 804, 806, 808, 810, 812, 814, 816, 818,and 820. FIG. 8B illustrates an example of a set of simulated patterns850 that may be produced from shot group 800, in the presence ofmid-range scattering. Set of patterns 850 comprises six whole patterns,including pattern 852, pattern 854, pattern 856, pattern 858, pattern860 and pattern 862. Pattern group 850 also comprises four additionalpatterns where only a portion of the pattern is illustrated in FIG. 8B,including pattern 864, pattern 866, pattern 868 and pattern 870. Thepatterns in pattern group 850 exhibit corner rounding due to beam blur,one example of which is corner 872. Additionally, the middle portion,measured in the y-direction, of each pattern in the middle two columnsis narrower in the x-direction than is the rest of the pattern, asillustrated by middle portion 874 of pattern 858. This narrowing is theresult of less mid-range scattering energy reaching the middle portion874 of pattern 858 than reaches other portions of pattern 858. Inpattern 858, pattern narrowing in region 874 is caused by the gapbetween shots 814 and 806, and by the gap between shots 818 and 812.Less mid-range scattering energy reaches the resist in the vicinity ofpattern 858 opposite these gaps, compared to opposite shots 814, 806,818 and 812. Outside column patterns 852, 854, 868, 862 and 870 exhibitasymmetrical narrowing because of their having neighboring shots on onlyone of the left or right sides. Inward-facing sides have a similarnarrowing as pattern 858, as illustrated with narrowing region 876 ofpattern 862. On outside-facing edges such as edge 878 of pattern 862 thelack of a neighboring pattern causes lower mid-range scattering energyto be received along the entire edge, with the consequence that theentire edge 878 is offset in the −x (negative x) direction, causing thewidth 882 of pattern 862 to be less than width 880 of pattern 858. Thissimulated midrange scattering is similar in range of effect to themidrange scattering of reticles for EUV optical lithography, but themidrange scattering simulated in pattern group 850 is of a higherintensity than current EUV reticles commonly produce. Pattern group 850illustrates how mid-range scattering of a sufficient magnitude canaffect patterns written by charged particle beam lithography.

In another embodiment of the current invention, overlapping shots may beused to implement mask process correction, thereby producing higherfidelity patterns in the presence of mid-range scattering. FIG. 9Aillustrates a shot group 900 that may be used to produce the group ofpatterns 800. Shot group 900 comprises rectangular shots 902, 904, 906,908, 910 and 912. Shot group 900 also comprises rectangular shots 914,916, 918 and 920, only portions of which are illustrated. Compared toshot group 800, shot group 900 includes the following:

-   -   Shots on the outside columns are widened on their outside edges.        This includes shots 902, 904, 918, 912 and 920. In shot 912, for        example, edge 936 has been moved in the +x direction, compared        to shot 812.    -   Additional shots are added to prevent the pattern narrowing in        the middle portion of the patterns as illustrated in pattern        group 850. The added shots include shots 922, 924, 926, 928, 930        and 932. These added shots deliver additional dosage to areas,        with the exception of outside edges of outside column shots,        that will receive less mid-range scattering dosage. Since on the        outside columns of shot group 900, pattern narrowing is        prevented by widening shots 902, 904, 918, 912 and 920 on their        outside edges as described above, overlapping shots 922, 924 and        932 are positioned away from the outside edges of shots 902, 904        and 912 to prevent excessive middle-portion widening of the        patterns formed by shots 902, 904 and 912.

FIG. 9B illustrates an example of a group of patterns 950 that may beproduced on a surface from group of shots 900. Group of patterns 950comprises patterns 952, 954, 956, 958, 960 and 962, and partial patterns964, 966, 968 and 970. As can be seen, the exposure changes illustratedin group of shots 900 compared to group of shots 800 improve thefidelity of the patterns produced on the surface, in the presence ofmid-range scattering. Narrowing of the middle portions of patterns isabsent. Additionally, the widths of exterior column patterns, such aswidth 982 of pattern 962, are the same as the widths of interior columnpatterns, such as width 980 of pattern 958.

The calculations described or referred to in this invention may beaccomplished in various ways. Generally, calculations may beaccomplished by in-process, pre-process or post-process methods.In-process calculation involves performing a calculation when itsresults are needed. Pre-process calculation involves pre-calculating andthen storing results for later retrieval during a subsequent processingstep, and may improve processing performance, particularly forcalculations that may be repeated many times. Calculations may also bedeferred from a processing step and then done in a later post-processingstep. An example of pre-process calculation is a shot group, which is apre-calculation of dosage pattern information for one or more shotsassociated with a given input pattern or set of input patterncharacteristics. The shot group and the associated input pattern may besaved in a library of pre-calculated shot groups, so that the set ofshots comprising the shot group can be quickly generated for additionalinstances of the input pattern, without pattern re-calculation. In someembodiments, the pre-calculation may comprise simulation of the dosagepattern that the shot group will produce on a resist-coated surface. Inother embodiments, the shot group may be determined without simulation,such as by using correct-by-construction techniques. In someembodiments, the pre-calculated shot groups may be stored in the shotgroup library in the form of a list of shots. In other embodiments, thepre-calculated shot groups may be stored in the form of computer codethat can generate shots for a specific type or types of input patterns.In yet other embodiments, a plurality of pre-calculated shot groups maybe stored in the form of a table, where entries in the table correspondto various input patterns or input pattern characteristics such aspattern width, and where each table entry provides either a list ofshots in the shot group, or information for how to generate theappropriate set of shots. Additionally, different shot groups may bestored in different forms in the shot group library. In someembodiments, the dosage pattern which a given shot group can produce mayalso be stored in the shot group library. In one embodiment, the dosagepattern may be stored as a two-dimensional (X and Y) dosage map called aglyph.

FIG. 10 is a conceptual flow diagram 1050 of how to prepare a reticlefor use in fabricating a surface such as an integrated circuit on asilicon wafer. In a first step 1052, a physical design, such as aphysical design of an integrated circuit, is designed. This can includedetermining the logic gates, transistors, metal layers, and other itemsthat are required to be found in a physical design such as that in anintegrated circuit. The physical design may be rectilinear, partiallycurvilinear, or completely curvilinear. Next, in a step 1054, opticalproximity correction is determined. In an embodiment of this disclosure,this can include taking as input a library of pre-calculated shot groupsfrom a shot group library 1074. This can also alternatively, or inaddition, include taking as input a library of pre-designed characters1080 including complex characters that are to be available on a stencil1084 in a step 1062. In an embodiment of this disclosure, an OPC step1054 may also include simultaneous optimization of shot count or writetimes, and may also include a fracturing operation, a shot placementoperation, a dose assignment operation, or may also include a shotsequence optimization operation, or other mask data preparationoperations, with some or all of these operations being simultaneous orcombined in a single step. The OPC step may create partially orcompletely curvilinear patterns. The output of the OPC step 1054 is amask design 1056.

Mask process correction (MPC) 1057 may optionally be performed on themask design 1056. MPC modifies the pattern to be written to the reticle,compensating for effects such as the narrowing of patterns which areless than about 100 nm wide. In a step 1058, a mask data preparation(MDP) operation which may include a fracturing operation, a shotplacement operation, a dose assignment operation, or a shot sequenceoptimization may take place. MDP may use as input the mask design 1056or the results of MPC 1057. In some embodiments of the presentinvention, MPC may be performed as part of a fracturing or other MDPoperation. Other corrections may also be performed as part of fracturingor other MDP operation, the possible corrections including: forwardscattering, resist diffusion, Coulomb effect, etching, backwardscattering, fogging, loading, resist charging, and EUV midrangescattering. The result of MDP step 1058 is a shot list 1060. Either theOPC step 1054 or of the MDP step 1058, or a separate program 1072 caninclude pre-calculating one or more shot groups that may be used for agiven input pattern, and storing this information in a shot grouplibrary 1074. Combining OPC and any or all of the various operations ofmask data preparation in one step is contemplated in this disclosure.Mask data preparation step 1058, which may include a fracturingoperation, may also comprise a pattern matching operation to matchpre-calculated shot groups to create a mask that matches closely to themask design. Mask data preparation may also comprise reducing thesensitivity of the pattern written in step 1062 to manufacturingvariation. Mask data preparation may also comprise inputting patterns tobe formed on a surface with the patterns being slightly different,selecting a set of characters to be used to form the number of patterns,the set of characters fitting on a stencil mask, the set of characterspossibly including both complex and VSB characters, and the set ofcharacters based on varying character dose or varying character positionor varying the beam blur radius or applying partial exposure of acharacter within the set of characters or dragging a character to reducethe shot count or total write time. A set of slightly different patternson the surface may be designed to produce substantially the same patternon a substrate. Also, the set of characters may be selected from apredetermined set of characters. In one embodiment of this disclosure, aset of characters available on a stencil in a step 1080 that may beselected quickly during the mask writing step 1062 may be prepared for aspecific mask design. In that embodiment, once the mask data preparationstep 1058 is completed, a stencil is prepared in a step 1084. In anotherembodiment of this disclosure, a stencil is prepared in the step 1084prior to or simultaneous with the MDP step 1058 and may be independentof the particular mask design. In this embodiment, the charactersavailable in the step 1080 and the stencil layout are designed in step1082 to output generically for many potential mask designs 1056 toincorporate patterns that are likely to be output by a particular OPCprogram 1054 or a particular MDP program 1058 or particular types ofdesigns that characterizes the physical design 1052 such as memories,flash memories, system on chip designs, or particular process technologybeing designed to in physical design 1052, or a particular cell libraryused in physical design 1052, or any other common characteristics thatmay form different sets of slightly different patterns in mask design1056. The stencil can include a set of characters, such as a limitednumber of characters that was determined in the step 1058.

The shot list 1060 is used to generate a surface in a mask writing step1062, which uses a charged particle beam writer such as an electron beamwriter system. Mask writing step 1062 may use stencil 1084 containing aplurality of complex characters, or may use a stencil comprising onlyVSB apertures. The electron beam writer system projects a beam ofelectrons through the stencil onto a surface to form patterns in asurface, as shown in a step 1064. The completed surface may then be usedin an optical lithography machine, which is shown in a step 1066.Finally, in a step 1068, a substrate such as a silicon wafer isproduced. As has been previously described, in step 1080 characters maybe provided to the OPC step 1054 or the MDP step 1058. The step 1080also provides characters to a character and stencil design step 1082 ora shot group generation step 1072. The character and stencil design step1082 provides input to the stencil step 1084 and to the characters step1080. The shot group generation step 1072 provides information to theshot group library 1074. Also, a shot group pre-calculation step 1072may use as input the physical design 1052 or the mask design 1056, andmay pre-calculate one or more shot groups, which are stored in a shotgroup library 1074.

Model-based fracturing may be combined with conventional fracturing in asingle design. This allows, for example, model-based fracturing to beused in those areas where it can provide the greatest benefit, whileusing conventional fracturing, which is less computationally intensive,for other parts of the design. As previously indicated, in conventionalfracturing, shot overlap is avoided whenever possible, and all shotshave a normal dosage before long-range correction. In FIG. 11Aconceptual flow diagram 1100 illustrates one embodiment for howconventional and model-based fracturing may be combined. The input tothe combined fracturing process is mask design 1102. Mask design 1102may be mask design 1056 from FIG. 10, or it may be a part of mask design1056, or an altered form of mask design 1056 such as from MPC 1057.Conventional fracturing 1104 is performed on the mask design 1102 tocreate a conventional shot list 1106. Alternatively, conventionalfracturing may be performed on parts of the mask design 1102, leavingsome parts unfractured. A model-based fracturing step 1108 then inputsthe shot list 1106 and modifies, adds, or deletes shots in complex areasof a design. Complex areas may include, for example, areas with thesmallest patterns, or areas with curvilinear patterns. Complex areas mayalso include areas with high influence from midrange scattering. Complexareas may also include “hot spots” of particular sensitivity inmanufacturing. The word “complex” in this context may not mean geometriccomplexity of the shapes. In some embodiments, the model-basedfracturing 1108 may include determining in which areas to modify and/orreplace conventional shots with model-base shots. In other embodimentsthe complex areas may be determined in a separate step 1112, eitherautomatically from mask design 1056, or manually. In any case,model-based fracturing 1108 generates shots, some of which partiallyoverlap other shots. The model-based fracturing may replace or modifysome or all of the conventional shots in the designated or determinedcomplex portions of the design with shots that have been determinedusing model-based techniques. The output of the model-based fracturingstep 1108 is a final shot list 1110, containing both conventional andmodel-based shots. Final shot list 1108 corresponds to FIG. 10 shot list1060. With regard to coarse grain parallel processing of the steps inconceptual flow diagram 1100, the mask design 1102 may be a partialdesign, or it may be the entire design where each of the steps may beperformed in parallel.

FIG. 11B conceptual flow diagram 1120 illustrates another embodiment ofhow conventional and model-based fracturing may be combined. The inputto the combined fracturing process is mask design 1122. Mask design 1122may be mask design 1056 from FIG. 10, or it may be a part of mask design1056, or an altered form of mask design 1056 such as from MPC 1057. InFIG. 11B the mask design 1122 is processed by pattern division step1124, which separates the pattern data into non-complex pattern area1126 and complex pattern area 1128. A conventional fracturing step 1130uses the non-complex pattern area 1126 as input. The conventionalfracturing 1130 outputs a list of conventional shots 1136. An additionaloutput is PEC information 1132. In some embodiments this information maybe one or more forms directly usable by PEC. In other embodiments, thePEC information may be, for example, the conventional shot list itself,from which PEC information may be calculated. The complex pattern area1128 is fractured using model-based fracturing 1134. Model-basedfracturing 1134 may use the PEC information 1132 as input, processingthis information if necessary to derive the appropriate PEC correctionsfor the model-based shots which are within the influence range of thelong-range effects from the conventional shots. In other embodiments,the PEC information may also be output by model-based fracturing 1134,and conventional fracturing 1130 may use this information in some way.Model-based fracturing 1134 creates a model-based shot list 1138. Theconventional shot list 1136 and the model-based shot list 1138 are thenmerged into a merged shot list 1140, which corresponds to FIG. 10 shotlist 1060. With regard to coarse grain parallel processing of the stepsin conceptual flow diagram 1120, the mask design 1122 may be a partialdesign, or it may be the entire design where each of the steps may beperformed in parallel.

The fracturing, mask data preparation, proximity effect correction andshot group creation flows described in this disclosure may beimplemented using general-purpose computers with appropriate computersoftware as computation devices. Due to the large amount of calculationsrequired, multiple computers or processor cores may also be used inparallel. In one embodiment, the computations may be subdivided into aplurality of 2-dimensional geometric regions for one or morecomputation-intensive steps in the flow, to support parallel processing.In another embodiment, a special-purpose hardware device, either usedsingly or in multiples, may be used to perform the computations of oneor more steps with greater speed than using general-purpose computers orprocessor cores. In one embodiment, the special-purpose hardware devicemay be a graphics processing unit (GPU). In another embodiment, theoptimization and simulation processes described in this disclosure mayinclude iterative processes of revising and recalculating possiblesolutions, so as to minimize either the total number of shots, or thetotal charged particle beam writing time, or some other parameter. Inyet another embodiment, an initial set of shots may be determined in acorrect-by-construction method, so that no shot modifications arerequired.

While the specification has been described in detail with respect tospecific embodiments, it will be appreciated that those skilled in theart, upon attaining an understanding of the foregoing, may readilyconceive of alterations to, variations of, and equivalents to theseembodiments. These and other modifications and variations to the presentmethods for fracturing, mask data preparation, proximity effectcorrection and optical proximity correction may be practiced by those ofordinary skill in the art, without departing from the spirit and scopeof the present subject matter, which is more particularly set forth inthe appended claims. Furthermore, those of ordinary skill in the artwill appreciate that the foregoing description is by way of exampleonly, and is not intended to be limiting. Steps can be added to, takenfrom or modified from the steps in this specification without deviatingfrom the scope of the invention. In general, any flowcharts presentedare only intended to indicate one possible sequence of basic operationsto achieve a function, and many variations are possible. Thus, it isintended that the present subject matter covers such modifications andvariations as come within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A method for fracturing or mask data preparationor mask process correction or proximity effect correction for chargedparticle beam lithography, the method comprising a step of: determininga plurality of charged particle beam shots that will form a pattern on asurface using a shaped beam charged particle beam writer, wherein atleast two charged particle beam shots in the plurality of chargedparticle beam shots partially overlap, and wherein sensitivity of thepattern on the surface to manufacturing variation is reduced, andwherein the step of determining is performed using one or more computinghardware processors, and wherein the sensitivity to manufacturingvariation is reduced by increasing edge slope, compared to use ofnon-overlapping normal dosage variable shaped beam (VSB) shots to formthe pattern.
 2. The method of claim 1 wherein the step of determiningcomprises calculating the pattern on the surface from the plurality ofcharged particle beam shots.
 3. The method of claim 2 wherein thecalculation comprises charged particle beam simulation.
 4. The method ofclaim 3 wherein the charged particle beam simulation includes at leastone of a group of short-range effects consisting of forward scattering,resist diffusion, Coulomb effect, and etching.
 5. The method of claim 3wherein the surface is an extreme ultraviolet (EUV) reticle, and whereinthe charged particle beam simulation includes EUV mid-range scattering.6. The method of claim 3 wherein the charged particle beam simulationincludes at least one of a group of long-range effects consisting ofbackward scattering, fogging, loading and resist charging.
 7. The methodof claim 1 wherein each charged particle beam shot in the plurality ofcharged particle beam shots comprises an assigned dosage, and whereinthe assigned dosage of a first charged particle beam shot in theplurality of charged particle beam shots is different from the assigneddosage of a second charged particle beam shot in the plurality ofcharged particle beam shots, before long-range correction.
 8. The methodof claim 1 wherein in the step of determining, the pattern on thesurface is a first pattern, the method further comprising the step ofgenerating a set of non-overlapping normal dosage variable shaped beam(VSB) shots that will form a second pattern on the surface, wherein thefirst pattern and the second pattern are adjacent.
 9. The method ofclaim 8 wherein short-range and/or long-range effects from the set ofnon-overlapping VSB shots are used in the step of determining.
 10. Themethod of claim 1 wherein charged particle beam shots in the pluralityof charged particle beam shots are variable shaped beam (VSB) shots. 11.The method of claim 1 wherein the plurality of charged particle beamshots is exposed in a single exposure pass.
 12. The method of claim 1,further comprising the steps of: inputting a set of non-overlappingvariable shaped beam (VSB shots of normal dosage that will form thepattern on the surface; and replacing some or all VSB shots in the setof VSB shots with the determined plurality of charged particle beamshots.
 13. A method for fracturing or mask data preparation or maskprocess correction or proximity effect correction for charged particlebeam lithography, the method comprising steps of: inputting a desiredpattern to be formed on a surface; and determining a plurality ofcharged particle beam shots that will form the pattern on the surfaceusing a shaped beam charged particle beam writer, wherein at least twocharged particle beam shots in the plurality of charged particle beamshots partially overlap, and wherein the plurality of charged particlebeam shots will form a pattern on the surface which is closer to thedesired pattern, compared to a pattern formed from non-overlappingnormal dosage variable shaped beam (VSB) shots, and wherein the step ofdetermining is performed using one or more computing hardwareprocessors, and wherein the sensitivity to manufacturing variation isreduced by increasing edge slope, compared to use of non-overlappingnormal dosage VSB shots to form the pattern.
 14. The method of claim 13wherein the step of determining comprises calculating the pattern on thesurface from the plurality of charged particle beam shots.
 15. Themethod of claim 14 wherein the calculation comprises charged particlebeam simulation.
 16. A system for fracturing or mask data preparation ormask process correction or proximity effect correction for chargedparticle beam lithography comprising: a device for determining aplurality of charged particle beam shots that will form a pattern on asurface, wherein at least two charged particle beam shots in theplurality of charged particle beam shots partially overlap, and whereinsensitivity of the pattern on the surface to manufacturing variation isreduced, and wherein the sensitivity to manufacturing variation isreduced by increasing edge slope, compared to use of non-overlappingnormal dosage variable shaped beam (VSB) shots to form the pattern.